/*

 * Licensed to the Apache Software Foundation (ASF) under one

 * or more contributor license agreements.  See the NOTICE file

 * distributed with this work for additional information

 * regarding copyright ownership.  The ASF licenses this file

 * to you under the Apache License, Version 2.0 (the

 * "License"); you may not use this file except in compliance

 * with the License.  You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

package com.bff.gaia.unified.sdk.transforms;



import com.bff.gaia.unified.sdk.values.TypeDescriptor;

import com.bff.gaia.unified.sdk.coders.Coder;



import javax.annotation.Nullable;

import java.lang.reflect.Method;



/**

 * A {@link SerializableFunction} which is not a <i>functional interface</i>. Concrete subclasses

 * allow us to infer type information, which in turn aids {@link Coder

 * Coder} inference.

 */

public abstract class SimpleFunction<InputT, OutputT> extends InferableFunction<InputT, OutputT>

    implements SerializableFunction<InputT, OutputT> {



  @Nullable

  private final SerializableFunction<InputT, OutputT> fn;



  protected SimpleFunction() {

    this.fn = null;

    // A subclass must override apply if using this constructor. Check that via

    // reflection.

    try {

      Method methodThatMustBeOverridden =

          SimpleFunction.class.getDeclaredMethod("apply", Object.class);

      Method methodOnSubclass = getClass().getMethod("apply", Object.class);



      if (methodOnSubclass.equals(methodThatMustBeOverridden)) {

        throw new IllegalStateException(

            "Subclass of SimpleFunction must override 'apply' method"

                + " or pass a SerializableFunction to the constructor,"

                + " usually via a lambda or method reference.");

      }



    } catch (NoSuchMethodException exc) {

      throw new RuntimeException("Impossible state: missing 'apply' method entirely", exc);

    }

  }



  protected SimpleFunction(SerializableFunction<InputT, OutputT> fn) {

    this.fn = fn;

  }



  @Override

  public OutputT apply(InputT input) {

    return fn.apply(input);

  }



  public static <InputT, OutputT>

  SimpleFunction<InputT, OutputT> fromSerializableFunctionWithOutputType(

	  SerializableFunction<InputT, OutputT> fn, TypeDescriptor<OutputT> outputType) {

    return new SimpleFunctionWithOutputType<>(fn, outputType);

  }



  /**

   * A {@link SimpleFunction} built from a {@link SerializableFunction}, having a known output type

   * that is explicitly set.

   */

  private static class SimpleFunctionWithOutputType<InputT, OutputT>

      extends SimpleFunction<InputT, OutputT> {



    private final TypeDescriptor<OutputT> outputType;



    public SimpleFunctionWithOutputType(

		SerializableFunction<InputT, OutputT> fn, TypeDescriptor<OutputT> outputType) {

      super(fn);

      this.outputType = outputType;

    }



    @Override

    public TypeDescriptor<OutputT> getOutputTypeDescriptor() {

      return outputType;

    }

  }

}